The present invention relates to thermoplastic elastomer compositions particularly useful for tire and other industrial rubber applications and processes for producing such compositions.
EP722850B1 discloses a low-permeability thermoplastic elastomer composition that is superior as a gas-barrier layer in pneumatic tires. This thermoplastic elastomer composition comprises a low-permeability thermoplastic matrix, such as polyamide or a blend of polyamides, in which there is dispersed a low-permeability rubber, such as brominated poly(isobutylene-co-paramethylstyrene), referred to hereinafter as BIMSM. In EP857761A1 and EP969039A1, the viscosity ratio of the thermoplastic matrix and the dispersed rubber phase was specified both as a function of the volume fraction ratio and, independently, to be close to a value of one in order to produce a high concentration of small particle size vulcanized rubber particles dispersed in a thermoplastic phase. EP969039A1 further discloses that small particle size rubber dispersed in a thermoplastic resin matrix was important in order to achieve acceptable durability of the resulting composition, particularly where such compositions are intended to be used as innerliners in pneumatic tires.
Compositions exhibiting low gas permeability performance (i.e., functioning as a gas barrier) composed of thermoplastic resin/thermoplastic resin-based blends such as a high density polyethylene resin and nylon 6 or nylon 66 (HDPE/PA6.66), a polyethylene terephthalate and aromatic nylon (PET/MXD6), a polyethylene terephthalate and vinyl alcohol-ethylene copolymer (PET/EVOH), where one thermoplastic resin is layered over the other layer to form plural layers by molding, and processes for producing the same have been proposed. An application regarding the use of such a composition as the innerliner layer of a tire is disclosed in Japanese Patent Application No. 7-55929. However, since these materials are thermoplastic resin/thermoplastic resin blends, while they are superior in gas barrier performance, they lack flexibility, and therefore, such films are subject to failure if they are used in a vehicle tire which is subject to significant flexing.
In many of the known thermoplastic elastomeric materials that are obtained via dynamic vulcanization (i.e. DVAs), to disperse the minor component in the DVA process wherein the minor component of the blend forms the continuous domain in the DVA and to facilitate extrusion manufacturing processes, a relatively high level of plasticizer (as a fraction of the nylon component, and in relation to the amount of plasticizer typically employed in plasticized nylons) has been used in the DVA compositions. However, a high level of plasticizer may not be desirable for the end product as the excess plasticizer may leach to the surface of the material and cause problems in storage of the unprocessed material, in extrusion, and in subsequent processing of the film. Residual plasticizer may also reduce impermeability characteristics of the material, reducing its effectiveness for use as a barrier material.
Past attempts to address this issue have included reduction of the plasticizer; however, the material must still be readily converted to a film using conventional extrusion processes. Even the remaining low levels of plasticizer may still have leaching issues, as well as some volatizing of the plasticizer during the processing. Capture of volatized plasticizer is possible, but is not an easy process and requires retrofitting for manufacturing. Additionally, any process involving capture of volatized plasticizer must take into consideration if the DVA is co-extruded with an adhesive material. In such a process, the adhesive must be tolerant of the drying conditions or it will require the adhesive to be applied as a separate operation after the film has been dried.
The inventors have observed that the stiffness of a DVA melt increases with time and temperature under stagnant conditions and decreases under some straining conditions. This is believed to be an undesirable characteristic for film conversion as it leads to divergence of the melt properties in an extrusion system that has a distribution of residence times and strain rates, and involves free-surface flows. If the ‘stagnation-stiffening’ is due to forces between the rubber particles, the effect can be anticipated to be more pronounced when the volume ratio of rubber to plastics in the DVA material is higher, as would be the situation when the amount of plasticizer is reduced in the DVA formulation.
The fundamental reason that conventional extrusion processes cannot readily be used with pellets that contain very reduced or no plasticizer is that, even if the material can be extruded, the viscosity of the material is so high, that when combined with the extrusion process, the material is degraded due to shear heating; one alternative is extrusion at uneconomically slow rates. The pressure of the melt acts on the full cross-sectional area of the extruder barrel so the forces developed in the attachment between extruder and downstream hardware become excessive. Also, in a blown film stacked die configuration the forces developed between the die elements increase as the second power of the die diameter, which means that high melt viscosity and the high melt pressure resulting may limit the layflat dimension of the bubble. This type of die is preferred for simultaneous extrusion of DVA and adhesive. Lastly, passage of the melt through a narrow die gap leads to excessively high pressures or unacceptably low throughput rates. Larger die gaps are not possible because the material has insufficient draw-down capability.
In using DVAs as barrier materials, especially as tire innerliners, the DVA material must provide an optimum balance of barrier properties and low temperature fatigue life. Fatigue life is improved as the rubber particle size in the thermoplastic resin domain is reduced. However, the particle size typical in DVA extrusion is relatively insensitive to process conditions. It is also believed that the elongation and possible orientation of the film structure in the film conversion process and tire building improve the barrier property, but although the melt undergoes a very high draw-down, the rubber particles are substantially less elongated, so it is possible a more significant improvement in the balance of fatigue life and barrier properties may be achieved if the rubber particles could be reduced in size and more oriented than is possible with the forces, times, and temperatures that are practical in a conventional film extrusion process.